AQA GCSE Physics

Revision Notes

5.9.3 Collisions

Higher Tier Only


  • Examples of momentum in an event are collisions
  • Objects will either:
    • Collide and move in opposite directions – this is an elastic colision
    • Collide and move in the same direction together – this is an inelastic collision
  • When the objects move in opposite directions:
    • Each object will have a different velocity depending on its mass and initial momentum of the system
  • When the objects move in the same direction together:
    • They will have a combined mass and velocity
  • Momentum is always conserved in a collision

Elastic & Inelastic Collisions, downloadable AS & A Level Physics revision notes

Types of collisions

Exam Tip

If an exam question asks you to analyse a collision, follow these tips for full marks:

  • Always consider the motion before and after the collision and state:
    • The velocities of each object
    • The direction each object moves
  • State whether the collision was elastic or inelastic and explain your reasoning
    • In a perfectly elastic collision, the kinetic energy is the same before and after
    • In a perfectly inelastic collision, the two objects stick together after colliding
  • Describe any energy transfers that occur if kinetic energy is not conserved
    • For example, it may be converted into heat, sound, elastic potential energy etc
Higher Tier Only

Calculations Involving Collisions

  • Calculations involving collisions use the conservation of momentum to determine the velocity of an object (or objects) before or after the collision
  • This means the momentum before the collision must equal the momentum after the collision for momentum to be conserved

Worked Example

An object of mass 1 kg is travelling at 3 m/s when it collides with a heavier object of mass 2 kg. The two objects stick together and travel off as one.

Calculate the combined velocity of the objects after the collision.

Step 1: Draw a diagram

momentum-example, IGCSE & GCSE Physics revision notesm-example

Step 2: State the principle of conservation of momentum

The total momentum before a collision = The total momentum after a collision

Step 3: Calculate the momentum before the collision

    • Before the collision, only the 1 kg object has any momentum

pbefore = mv = 1 × 3 = 3 kg m/s

Step 3: Determine the momentum after the collision

    • The combined mass is now 1 + 2 = 3 kg

pafter = mv = 3 × v

Step 4: Substitute values into conservation equation

pbefore = pafter

3 = 3 × v

Step 5: Rearrange for the combined velocity v

v = 3 ÷ 3 = 1 m/s

Exam Tip

Always double-check the signs (positive or negative) for the velocity in your answers, as this is the most common type of calculation error!

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

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